docs: def get_proxy_ancestor_classes(klass):
      """
      Return a set containing all the proxy model classes that are ancestors
      of the given class.
  
      NOTE: This implementation is for Django 1.7, it might need to work
      differently for other versions especially 1.8+.
      """
      proxy_ancestor_classes = set()
      for superclass in klass.__bases__:
          if hasattr(superclass, '_meta') and superclass._meta.proxy:
              proxy_ancestor_classes.add(superclass)
          proxy_ancestor_classes.update(
              get_proxy_ancestor_classes(superclass))
      return proxy_ancestor_classes